THE 



CIVIL ENGINEER AND ARCHITECT'S 



JOURNAL. 



ON THE CONSTRUCTION OF THE MOLE DES NOIRE^^, 



WHICH SHELTERS THE FRONT HARBOUR AND 



ENTRANCE OF THE GATES OF ST. MALO. 



(With an Engraving, Plate I.) 



( Ti-amlated from the French vf M. Girard de Caudemherg, Engineer- 

 in-Chief of Roads and Bridges.) 



The Mole ties Noires, forming part of the general plan of a floating 

 basin which is to be common to St. Malo and St. Servan, has been in 

 progress for the last two vears, and is situated as pointed out in fig. 1, 

 stretching from A to B. When the wind bows from S.W. to N.W., 

 Jt is very much exposed to the action of the sea, and was consequently 

 during its construction exposed to all the most unfavourable contin- 

 gencies, by which works in direct contact with the sea are affected. 

 For the purpose of opposing this action, a form has been given to the 

 mole of an arc of a circle of 695 feet (212 met.) radius. The breadth 

 of the top, including the parapet, is 19 feet (5-SO uiet.j, which is 

 strictly necessary for preserving a free passage for warping to the 

 upper part, and for giving to the works the stability and resistance 

 necessary to support the difference of pressure resulting from the 

 maximum of the simultaneous elevation and depression of the waves 

 on the two opposite faces. The dimensions of the mole are given in 

 the section, fig. 2, in which are also shown the high and low water 

 marks at spring and neap tides, which sufficiently justify the great 

 elevation given to the work. This section also shows the great aque- 

 duct or interior tunnel, and of the channels communicating with it. 

 The aqueduct extends the whole length of the mole up to the head, 

 and the upper and lower channels or pipes are made at every 0,5 feet 

 distance. The lower inclined pipes end in a number of sluices, which 

 are for the purpose of clearing away the silt in the front harbour. 



The opinion of M. de Caudemherg was that this silt was little to be 

 feared, but as the commissioners appointed by the Minister of Marine, 

 insisted upon having an aqueduct which could work the sluices, in the 

 front harbour, M. de Caudemherg suggested the plan now in execution. 

 The aqueduct is 197S feet (1503 met.) in length and 7 feet 2 inches 

 (2'20 met.) diameter, and is carried through the mass of the quay of 

 the front harbour, crosses the gates of the inner harbour, and takes its 

 rise in the floating I asin. It is constructed throughout of an annular 



No. 40.— Vol. IW-Janvary, 1841. 



form. The pipes carrying the water to the sluices are inclined as re- 

 presented in the section, and are of cast iron, their inner diameter is 

 about IG inches (0-4U met), and the different parts which compose 

 them are secured by a simple joint with resinous mastic. The upper 

 part is terminated by a hemispherical cup IS inches diameter, with a 

 ball acting as a bomb valve. This valve is for the purpose of prevent- 

 ing the water introduced into the great aqueduct returning back again 

 when the tide falls. Where however the sluices are intended to be 

 worked the valve is lifted up, by means of a chain comnuuiicating witli 

 the upper surface of the quay. The water which flows from the aque- 

 duct, through the inclined pipes, with the velocity of a column of 

 water 24 feet high is carried through an opening 3 feet 3 inches broad, 

 and of a mean height of 5 inches, so as to cause a stream of water to 

 sweep away the silt. It should be observed that the large vertical 

 pipes 19 inches (4 met.) diameter, serve for the evacuation of the air, 

 and as manholes for cleansing and repairing the aqueduct. The parts 

 marked a 6 e in fig. 2, are for the purpose of preventing the water at 

 high tides from getting through the sluices, and causing an inverse 

 pressure on the great aqueduct. 



The engineer found considerable difficulty while constructing the 

 mole, on accomit of the position of the great aqueduct, which as it 

 was necessarily built upon a centering, would in case of wood being 

 employed, have been soon blown up by the waves and destroyed, or at 

 least have had the mortar forced out and the work to begin over again. 

 To prevent this M. de Caudemherg directed his attention to the con- 

 struction of a peculiar centering or shield. This centering was of cast 

 iron, in moveable pieces, so that it should be readily managed in the 

 progress of the works. On the outside was fixed an arm to break the 

 power of the wave at the period of the shock, while at the same time 

 the specific weight of the centering prevented it from being carried 

 away. It is formed of panels weighing about a hundred weight each, 

 so that they could be easily moved. The whole shield was 20 feet 

 fS met.) long, and divided into 10 rings. 



M. De Caudemherg found that though by these means he broke the 

 shock of the wave, that the works were still liable to suffer on account 

 of the oscillations, particularly when the weather was rough, when 

 masses of compressed air were" forced into the great aqueduct, and so 

 up the vertical manholes, causing spouts of water 30 or 40 feet high. 

 These manholes however served greatly to modify the effects. As 



B 



